This invention relates to a level shifter which shifts or reduces a source voltage and produces a level shifted voltage. Specially, this invention relates to a level shifter which is formed within a semiconductor substrate together with an integrated circuit and which has a voltage dividing circuit.
Recently, an operating voltage of an integrated circuit tends to be lowered so as to reduce consumption of electric power with a rise of integrational rate and heating value of the integrated circuit. To this end, a level shifter has often been used which is formed within a semiconductor substrate together with such an integrated circuit to shift a source voltage.
With a reduction of the operating voltage, strict operational conditions are imposed on a semiconductor element of the integrated circuit. For example, when the operating voltage is higher than a designed voltage, the semiconductor element, such as a MOS transistor, deteriorates. When the operating voltage is lower than the designed voltage, an operating speed of the semiconductor element is reduced. This means that operating characteristics of the semiconductor element are easily influenced by an atmospheric temperature. In Japanese Patent Prepublication No. 196317/1991, proposal has been made about a conventional level shifter which has a predetermined temperature characteristic so as to alleviate or relieve an influence of the atmospheric temperature.
More specifically, the conventional level shifter has a voltage dividing circuit of two resistors which are connected to each other in series and which are farmed by a semiconductor integrated technique as integrated circuit elements. Such resistors will be called integrated-circuit resistors and have peculiar temperature characteristics in relation to the predetermined temperature characteristic. In the proposed level shifter, both the peculiar temperature characteristics of the integrated-circuit resistors are different from each other. Namely, the resistors have resistances which individually vary in dependency upon the atmospheric temperature. As a result, a divided voltage also varies in the dividing circuit in response to the atmospheric temperature. The conventional level shifter is operable to produce a level shifted voltage from the divided voltage. This shows that the level shifted voltage varies in dependency upon the atmospheric temperature.
In the conventional level shifter, both the integrated-circuit resistors should have different line widths because the integrated-circuit resistors establish peculiar temperature characteristics as mentioned before. This results in a large variation on manufacturing the level shifter. In addition, integrated-circuit resistors must be separately formed by different manufacturing process in order to have different temperature characteristics.
Therefore, the conventional level shifter can not produce a expected shifted source voltage.